LLVM libc clang-tidy checks

These are the clang-tidy checks designed to help enforce implementation standards. The configuration file is src/.clang-tidy.


One of libc-project’s design goals is to use kernel headers and compiler provided headers to prevent code duplication on a per platform basis. This presents a problem when writing implementations since system libc headers are easy to include accidentally and we can’t just use the -nostdinc flag. Improperly included system headers can introduce runtime errors because the C standard outlines function prototypes and behaviors but doesn’t define underlying implementation details such as the layout of a struct.

This check prevents accidental inclusion of system libc headers when writing a libc implementation.

#include <stdio.h>            // Not allowed because it is part of system libc.
#include <stddef.h>           // Allowed because it is provided by the compiler.
#include "internal/stdio.h"   // Allowed because it is NOT part of system libc.


It is part of our implementation standards that all implementation pieces live under the __llvm_libc namespace. This prevents pollution of the global namespace. Without a formal check to ensure this, an implementation might compile and pass unit tests, but not produce a usable libc function.

This check that ensures any function call resolves to a function within the __llvm_libc namespace.

// Correct: implementation inside the correct namespace.
namespace __llvm_libc {
    void LLVM_LIBC_ENTRYPOINT(strcpy)(char *dest, const char *src) {}
    // Namespaces within __llvm_libc namespace are allowed.
    namespace inner{
        int localVar = 0;
    // Functions with C linkage are allowed.
    extern "C" void str_fuzz(){}

// Incorrect: implementation not in a namespace.
void LLVM_LIBC_ENTRYPOINT(strcpy)(char *dest, const char *src) {}

// Incorrect: outer most namespace is not correct.
namespace something_else {
    void LLVM_LIBC_ENTRYPOINT(strcpy)(char *dest, const char *src) {}


LLVM-libc is distinct because it is designed to maintain interoperability with other libc libraries, including the one that lives on the system. This feature creates some uncertainty about which library a call resolves to especially when a public header with non-namespaced functions like string.h is included.

This check ensures any function call resolves to a function within the __llvm_libc namespace.

There are exceptions for the following functions: __errno_location so that errno can be set; malloc, calloc, realloc, aligned_alloc, and free since they are always external and can be intercepted.

namespace __llvm_libc {

// Allow calls with the fully qualified name.

// Allow calls to compiler provided functions.

// Bare calls are allowed as long as they resolve to the correct namespace.

// Disallow calling into functions in the global namespace.

// Allow calling into specific global functions (explained above)

} // namespace __llvm_libc